1. Field of the Invention
The present invention relates to internal combustion engines and, more particularly, to a two-stroke, over expanded homogeneous charge compression ignition (HCCI) engine cycle designed to solve the major obstacles preventing the commercialization of HCCI engines, including controlling the timing of autoignition and operation over a wide range of load requirements. In addition to solving these problems, the cycle provides superior thermal and mechanical efficiency over existing four-stroke HCCI engines.
2. Background of the Prior Art
In the quest to develop a better internal combustion engine, satisfying emissions requirements is paramount. Potential gains in power and/or efficiency are immaterial, unless a new engine design is able to meet such requirements in a commercially feasible way. Of the three primary noxious emissions (NOx, CO and HC), developing ways to reduce nitrogen oxide (NOx) emissions is perhaps the most vexing. Moreover, under current engine operating cycles, potential solutions that address NOx emissions tend to exacerbate carbon monoxide (CO) and hydrocarbon (HC) emissions.
In traditional diesel engines, the high temperature of the early burned fuel-air mixture is the primary culprit in NOx formation. More specifically, combustion is initiated by injecting fuel into compressed air and the temperature rises due to the burning of the fuel. Importantly, the combustion of the fuel results in expansion of the burning gases thereby causing a rapid increase in pressure. This rapid increase in pressure following combustion causes an additional increase in temperature of the already burned gas. The cumulative increase in temperature that results from the burning of the fuel-air mixture, plus the additional increase caused by the post-burning compression, is referred to herein as the “post-combustion temperature.” In a traditional diesel engine, the post-combustion temperature exceeds the threshold temperature at which unacceptable levels of NOx are formed.
Over the past several years, HCCI engines have held the promise of providing cleaner burning and more fuel efficient internal combustion engines. Characterized by the autoignition of a compressed lean homogenous charge, the entire compressed fuel/air mixture burns simultaneously avoiding further compression of already burned gases, which is the primary cause for the high combustion temperatures that cause the formation of NOx. Several obstacles, however, have thus far hindered the development of a commercially viable HCCI engine.
First, researchers have yet to develop a viable means for controlling the timing of autoignition of the compressed homogenous charge. Combustion is initiated by the compression of the homogenous charge to the required autoignition temperature. There is no commercially viable means, however, to precisely control the timing of autoignition because in a four-stroke HCCI cycle the chemical kinetics involved in the autoignition timing have thus far proved too complex to predict or control. In addition, even if the problem of controlling autoignition timing of the homogenous charge could be solved, conventional four-stroke HCCI engines can only sustain HCCI operation over a narrow range of load conditions.